Residual Fields in Superconducting Dipole and Quadrupole Magnets

This paper discusses the magnetic measurements of residual field in superconducting dipole and quadrupole magnets. The technique used to measure residual fields is described herein. The results of the measurement are presented and compared with a simplified theory, which predicts the basic nature of residual fields. The theory and experiment agree reasonably well despite the simplifying assumptions made in the theory. The residual fields generated in a superconducting magnet do affect the uniformity of the field generated in such a magnet. The effects of residual field are worst at low field, which is precisely where one wants the most magnet uniformity in a superconducting synchrotron. The theory suggests that one can reduce the magnitude by: (1) reducing the strand diameter in the material (smaller strands require more twist to eliminate coupling); and (2) reducing the low-field critical current of the material, which also reduces ac loss and instabilities. The theory also suggests that superconducting synchrotrons with injection inductions as low as 1 kG must have superconductor strands as small as 1 to 2 ¿m.